How to start

Start with a melody. A melody usually has two parts - the "question" and the "answer".

Make the bass line; it may be either just the root note of the first not in each bar or it may be a completely different and catchy bass line altogether.

Then, add the harmony. Try and visualize a story as you compose. The melody is usually the high part, the harmony is the middle part and the bass line is the low part of a song. On a practical level, knowing some theory can be useful to a composer in certain circumstances:

When a composition is not as interesting as the composer would like it to be. He/she would like to evoke something different than whatever the music inspires at that stage of composition. Examining the melody for how it conforms to standard scales, examining the harmony to detect whether or not it is feeling hidebound to predictable patterns and progressions, then contemplating how to mix it up with shifts in key feel, introducing certain accidentals - all this can be useful.
When one has composed a passage, and the harmony is not obvious. Analyzing the relationship between a melodic passage and all the possible ways it can be harmonized (Does it lie in a standard triad? Could it be an accidental?) can be done by theoretical knowledge.